The invention relates generally to dielectric resonator oscillators (DROs) and methods of their assembly and, more specifically, to DROs having dielectric resonators that are attached to the DRO housing in such a manner that the DRO operation is not substantially affected by mechanical vibration.
Dielectric resonator oscillators (DROs) are commonly used in high-precision RF and microwave systems to generate high-frequency signals of extremely good spectral purity. For example, DROs have been used in radars, transponders, and communication systems, among other systems, to generate microwave signals with extremely low phase noise and good temperature stability. Generally, in these systems, the DRO is used to generate a frequency that is locked to a reference oscillator within a phase-locked loop circuit.
FIG. 1 illustrates a cross-sectional, side view of a DRO 100 in accordance with the prior art. DRO 100 includes dielectric resonator 102, housing 104, housing lid 106, dielectric pedestal 108, printed wiring board substrate 110, microstrips 112, tuning screw 114, and wall mounted electric tuner 116. Dielectric resonator 102 is used as a frequency determining circuit element. Dielectric resonator 102 is made of a rigid ceramic material, having a very high dielectric value.
Housing 104 and housing lid 106 are made of a metallic material. Housing 104 is a structure having a bottom 118 and sides 120. When assembled, housing 104 and housing lid 106 create a resonant cavity 122.
During operation, electromagnetic energy is coupled into cavity 122 and resonator 102 at the resonant frequency via microstrips 112 located on substrate 110, which is located on the housing bottom 114. Likewise, energy at the resonant frequency can be extracted from cavity 122 via microstrips 112.
The relative position of dielectric resonator 102 within resonant cavity 122 affects the frequency characteristics and the Q of the DRO. The position of resonator 102 is defined by the height of pedestal 108 and the horizontal placement of dielectric resonator 102 on pedestal 108. Pedestal 108 is made of a solid, low-loss, low-dielectric constant material.
During a typical assembly process, substrate 110 and microstrips 112 are first attached to the housing bottom 118. Then, pedestal 108 is attached to the substrate 110 using an epoxy material, which requires a high-temperature, heat-curing process. In some prior art processes, pedestal 108 is attached directly to housing bottom 118 using an epoxy, after substrate 110 is attached to the housing bottom 118.
After pedestal 108 is attached to housing 104 (or substrate 110) and heat-cured, resonator 102 is placed on pedestal 108, and an iterative position adjustment process is performed. This is necessary because the oscillator circuit will oscillate only over a fairly narrow range of resonator positions on pedestal 108. The position adjustment process involves assembling housing lid 106 to housing 104, and testing the frequency. The lid 106 is then removed, and if the frequency is not accurate enough, the resonator position on pedestal 108 is adjusted along the horizontal plane. This testing and position adjustment process is repeated until the desired performance is attained. Resonator 102 is then carefully removed, applied with epoxy, and re-positioned on pedestal 108. The assembly is again heat-cured and tested for performance. Coarse frequency adjustments are then performed using tuning screw 114, as is described below.
As the above description indicates, the entire DRO 100 is heated at least twice during assembly. This double-heating process decreases the yield of acceptable DROs, because the circuitry within DRO 100 is cumulatively affected by the heating processes. In addition, it can be difficult and time consuming to accurately adjust the position of resonator 102 on pedestal 108, and to accurately re-position resonator 102 on pedestal 108 after application of the epoxy to resonator 102.
The dimensions of resonator 102 define the resonant frequency of resonator 102. This frequency can be varied by a small percentage using tuning screw 114 and/or electric tuner 116, both of which capacitively load resonator 102. Tuning screw 114 is used to coarsely tune resonator 102 (e.g., within about one percent of the resonant frequency), and wall mounted electric tuner 116 is used to finely tune resonator 102 (e.g., by tenths of a percent).
The frequency characteristics of resonator 102 are adversely affected if tuning screw 114 makes contact with resonator 102. Accordingly, an air gap 128 must exist between the bottom of tuning screw 114 and the top of resonator 102 in prior art systems.
In this prior art configuration, resonator 102 is held in position only by pedestal 108. Accordingly, when DRO 100 is subject to mechanical vibration, pedestal 108 and resonator 102 can sway. When the vibration is sufficient, the movement of resonator 102 within cavity 122 can be enough to adversely affect the frequency characteristics of the DRO 100. In some cases, the movement can be severe enough to cause frequency fluctuations in the megahertz range, which can cause a DRO that is used in conjunction with a phase-locked loop circuit to lose lock with the reference oscillator. If the vibration is more than momentary, the circuit will continue to lose lock. Because prior art DROs are so sensitive to resonator position, prior art DROs are unsuitable, in many cases, for use in mobile apparatus, or other apparatus that may experience vibration conditions.
What are needed are DROs and methods of their assembly that simplify the process of positioning the dielectric resonator within the DRO cavity. In addition, what are needed are methods of assembling a DRO that eliminate the need to subject the DRO circuitry to multiple heating processes. Finally, what are needed are DROs that have resonators mounted in a manner that the DRO operation is not substantially affected by mechanical vibration.